1. Field of the Invention
The present invention relates to a method and amplifier for cancelling magnetic coupling, and more particularly, to a method and amplifier capable of reducing in-phase/quadrature-phase (IQ) phase imbalance and gain imbalance, and minimizing layout area.
2. Description of the Prior Art
In communication systems, a carrier is frequently utilized for carrying baseband signals that contain data. Generally, a carrier is a high frequency signal. However, due to bandwidth limitation, a transmitter adopts a modulation scheme with high bandwidth efficiency. A quadrature amplitude modulation (QAM) is one of frequently utilized modulation schemes.
Generally, in a QAM system, signals are processed in two different paths. Ideally, the incoming signals are multiplied by an in-phase carrier (in-phase carrier) g sin wt and a quadrature-phase carrier g cos wt two mixers for modulation in the two paths, respectively, wherein g is the gain, and w is the angular frequency. In practice, factors such as magnetic coupling between inductors, temperature, process and supply voltage offset may result in gain imbalance and phase imbalance between the in-phase carrier g sin wt and the quadrature-phase carrier g cos wt, i.e. in-phase/quadrature-phase (IQ) imbalance. In other words, the oscillating signals utilized by the mixers may become (g+α)sin wt and g cos(wt+θ), where α is the gain imbalance and θ is the phase imbalance. In such a condition, there will be gain imbalance and phase imbalance between the mixed in-phase signal and the mixed quadrature-phase signal.
For example, please refer to FIG. 1A, which is a schematic diagram of a conventional quadrature amplifier 100. For clearly illustrating, the quadrature amplifier 100 shown in FIG. 1A only includes differential amplifiers 102, 104 and inductors 106, 108, while other components of the quadrature amplifier 100 are not shown. The inductors 106, 108 form inductors of two inductor capacitor (LC) tanks, respectively. The amplifier 102 and the inductor 106 are in an in-phase path (I path), and the amplifier 104 and the inductor 108 are in a quadrature-phase path (Q path), wherein a distance between the inductors 106 and 108 is H. In the quadrature amplifier 100, terminals IN and IP in the in-phase path output an in-phase negative signal S1 and an in-phase positive signal S2, respectively, and terminals QN and QP in the quadrature-phase path output a quadrature-phase negative signal S3 and a quadrature-phase positive signal S4, respectively. Ideally, a phase difference between the in-phase positive signal S2 and the quadrature-phase positive signal S4 is 90 degree. However, in order to keep carrier frequency within a range, inductors are utilized to achieve band-pass effect, and magnetic coupling between inductors may induce magnetic fields and generate corresponding induced currents, causing IQ imbalance.
For example, as shown in FIG. 1B to FIG. 1D, which are schematic diagrams of IQ phase imbalance between the in-phase positive signal S2 and the quadrature-phase positive signal S4 when the inductors 106 and 108 shown in FIG. 1A have different coil winding directions. In FIG. 1B, coil winding directions of the inductors 106 and 108 are clockwise, and thus magnetic field directions are downwards through paper. In such a situation, magnetic coupling between inductors having magnetic fields with a same direction generates induced currents, which shifts the in-phase positive signal S2 and the quadrature-phase positive signal S4 from original solid lines to dotted lines. Take a point A shown in FIG. 1B as an example, ideally, a phase of the in-phase positive signal S2 is 0 degree and a phase of the quadrature-phase positive signal S4 is 90 degree, such that a phase difference in between is 90 degree. However, after being affected by the induced currents generated by magnetic coupling between inductors having magnetic fields with the same direction, the phase of the in-phase positive signal S2 is greater than 0 degree and the phase of the quadrature-phase positive signal S4 is less than 90 degree, such that the phase difference in between is less than 90 degree, causing IQ phase imbalance.
Similarly, in FIG. 1D, coil winding directions of the inductors 106 and 108 are counterclockwise, and thus magnetic field directions are upwards through paper. Magnetic coupling between inductors having magnetic fields with a same direction generates induced currents, which results a phase difference between the in-phase positive signal S2 and the quadrature-phase positive signal S4 less than 90 degree, causing IQ phase imbalance. Similarly, in FIG. 1C, coil winding directions of the inductors 106 and 108 are clockwise and counterclockwise, respectively, and thus magnetic field directions are downwards through paper and upwards through paper, respectively. Take a point C shown in FIG. 1C as an example, after being affected by induced currents generated by magnetic coupling between inductors having magnetic fields with opposite directions, a phase of the in-phase positive signal S2 less than 0 degree and a phase of the quadrature-phase positive signal S4 is greater than 90 degree, such that a phase difference in between is greater than 90 degree, causing IQ phase imbalance.
In the prior art, in order to reduce IQ imbalance caused by magnetic coupling between inductors, the distance H between the inductors 106 and 108 is required to be very long, so as to reduce induced currents. As a result, large layout area is required, and magnetic coupling effect can not be totally eliminated, wherein the issue of IQ imbalance can not be effectively solved. Thus, there is a need for improvement of the prior art.